This application claims priority to Korean Patent Application Nos. 2003-0038322, filed on Jun. 13, 2003, and 2003-0066324, filed on Sep. 24, 2003, in the Korean Intellectual Property Office, the disclosures of which are incorporated herein in their entirety by reference.
1. Field of the Invention
The present invention relates generally to disc drives such as an optical disc drive, and more particularly, to a molded coil device for an actuator of disc drives for inducing an electromagnetic force upon interaction with a magnet.
2. Description of the Related Art
Typically, optical disc drives are devices which write or read information by radiating light on an optical disc, i.e., a recording medium. To this end, optical disc drives include a spindle motor which rotates an optical disc, and an optical pickup unit which performs write and reproduction operations by radiating light on a recording surface of the optical disc.
In general, the optical pickup unit includes an optical pickup actuator, which controls the position of an objective lens in focusing and tracking directions so that the focused light is formed on a desired track of the recording surface of the optical disc. The optical pickup actuator controls the objective lens to adjust a predetermined distance between the objective lens and the recording surface of the optical disc, for maintaining the focus of light spot(s) and for making the light spot(s) follow a desired track.
In order to perform precise write and reproduction operations, a function for dynamically adjusting tilt of the objective lens is also needed. Light emitted through the objective lens should be vertically incident on the recording surface of the optical disc so that light spot(s) having a precise focus is formed. If an incident direction of light is offset, precise light spot(s) cannot be formed on the optical disc. As such, errors may occur when writing and reproducing data. Adjustment for vertical incidence of light on the recording surface of the optical disc is referred to as tilt adjustment or skew adjustment. The optical pickup actuator may further include a mechanism for such tilt adjustment of the objective lens.
In the optical pickup actuator, coils are installed in a blade on which an objective lens is mounted. Magnets are installed in a base by which the blade is elastically and movably supported with the magnets facing the coils. When current flows through windings of a coil, an electromagnetic force is generated on the blade with the coil from the magnetic field of the magnets. The level of the electromagnetic force F is calculated by Lorentz's formula, F=BL×i, where B is the intensity of the magnetic field, L is the effective length of the coil, and i is the level of current flowing through the coil. A plurality of coils and magnets are disposed at appropriate positions so that the blade may be moved in the focusing direction, the tracking direction, and the tilting direction.
A coil used in the optical pickup actuator is conventionally a wire-wound coil. Prior art methods of forming a conventional wire-wound coil in a blade of an optical pickup include a bobbin wire-winding method and an alignment wire-winding method. In the bobbin wire-winding method, a wire is wound directly around a bobbin placed on the blade. In the alignment wire-winding method, a wire is wound around a separate bobbin, the bobbin is removed, and only the wound wire is installed on the blade.
Corner parts of the wire-wound coil tend to be wound around on a rectangular bobbin. Thus, in such corner parts, direction of the current is not perpendicular to the direction of the magnetic field. As the number of turns of the coils increases, adverse effect on the resulting electromagnetic field from such misalignment becomes significant. The number of turns of a wire-wound coil is desired to be maximized to maximize the effective length of the coil. However, the thickness of the wire is limiting to the possible number of turns. In addition, with increased number of turns of the wire, the size and weight of the coil is increased such that the driving sensitivity of the optical pickup actuator may be disadvantageously lowered. In addition, since the wound wire of the coil is exposed to the outside, when such an exposed coil contacts a yoke or any other conductive material during driving, the coil may be damaged and short-circuited.
Alternatively in the prior art, a flat coil is patterned on a printed circuit board (PCB). However, since the PCB is manufactured using a conventional method, even though the PCB is formed of multiple layers, the number of layers is limited to about 8 layers such that there is a limitation in increasing the number of turns of the coil. In addition, because portions of a flat coil may be exposed, the flat coil may come off from the PCB. Furthermore, a flat coil tends to have good low-frequency characteristics and bad high frequency characteristics.
As known from Lorentz's formula, the magnetic field of a magnet is desired to be perpendicular to the current of a coil. Since magnetic flux is bent farther from the magnet, the magnet and the coil are desired to be disposed as close as possible. On the other hand with such close disposition, the driving range of the blade is limited by interference between the coil and the magnet.
In addition, in order to make the magnetic flux linear, a yoke is typically installed in the base to face the magnet. For enhanced magnetic efficiency, it is preferable that the yoke be installed as close as possible to the coil. On the other hand with such close installation, the driving range of the blade is limited. Further, due to errors during assembling of the yoke and coil, the yoke and coil may interfere with each other to limit the driving range of the blade. In addition, the number of turns of a wire-wound coil is limited by the desired range of size and weight of the coil and from restrictions resulting from rounded corners of such a coil. As a result, increasing the effective length of the wire-wound coil or the flat coil patterned on a PCB for inducing an electromagnetic force is difficult.
Thus, a new type of coil is desired without such disadvantages of the prior art.